1. Field of the Invention
The present invention relates to a hydrophone with automatic inhibition when the immersion depth exceeds a certain adjustable threshold.
2. Description of the Prior Art
It is well-known to form hydrophones by assembling elements sensitive to pressure variations, such as discs made of a piezoelectric material, associated each with a pair of electrodes arranged on either side. Each disc is stuck onto a flexible support such as a diaphragm, one of the faces thereof being exposed to the pressure variations to be measured.
The flexible support has for example the shape of a cup which can be supported by a rigid housing or which can rest against an identical cup, itself carrying one or more sensitive elements, the two cups being in contact with one another at one edge and delimiting a housing. The electrodes of the two sensitive elements are preferably electrically interconnected so as to provide compensation for the parasitic effects due to accelerations.
When the outside static pressure increases, the two plates bend until they rest against each other. The space between them is so selected that their maximum deformation, when they are pressed against each other, remains within elastic deformation limits. The detector is thus protected against accidental overpressures. The housing thus formed can be coated with a layer made of a material transparent to acoustic waves.
The sensitive elements can be externally fastened to the housing so that their sensitivity does not vary much with the hydrostatic pressure variations (less than 10% for a 10 Map static pressure for example). The sensitive elements are generally covered with a protecting coating (such as a varnished araldite layer) so as to maintain a sufficient electric insulation in relation to the outside environment. This layout allows obtaining of very sensitive detectors at a relatively low cost.
According to another well-known layout, the sensitive elements are fastened to the inner faces of the cups and therefore inside the housing, which provides good protection against the outside environment. A stop can be arranged between the two cups so as to limit deformation of the diaphragms towards the inside and to short-circuit the electrodes of the two sensitive elements facing each other.
It is also well known to position the housings containing one or two sensitive elements inside a rigid tube by means of a hollowed flat centering element made of a deformable material, and to run an acoustically transparent sealed sheath therein. In subsea seismic listening applications or for seismic prospecting, these tubes are distributed in large numbers inside a supple sheath of often very great length, or streamer, filled with kerosine or mineral oil, which is towed by a boat
Various piezoelectric detectors are described for example in French patents 1,556,971, 2,122,675, 2,748,183 or 2,792,802 or U.S. Pat. No. 5,889,731, all filed in the name of one of the applicants or of both, or in U.S. Pat. Nos. 3,970,878, 4,336,639, 4,926,397 or 5,136,549.
Piezoelectric hydrophones, such as those mentioned above, are designed to work within a certain depth range. The sensitivity of the piezoelectric elements decreases with the flexion of the diaphragms. Beyond a certain flexion, their response to the pressure variations to be measured stops being reliable, and it can even become zero in case of a short-circuit of the electrodes facing each other (detectors with sensitive elements against the inner faces of the cups).
The automatic-inhibition hydrophone according to the invention is suited for use in a well-defined and adjustable depth range and is automatically inhibited when a predetermined depth threshold is exceeded. The hydrophone of the invention comprises at least one detection unit producing an electric signal on electric wires in response to the pressure variations applied.
The hydrophone of the invention comprises at least one flexible diaphragm that bends under the action of the pressure exerted externally on the hydrophone and a switch arranged opposite so as to be actuated by the diaphragm for a predetermined outside pressure, at least one of the electrical wires of the detection unit being connected to the switch so that the release of the switch inhibits the response of the sensitive element.
According to a first embodiment, the hydrophone comprises a tubular body comprising a fist chamber closed by a flexible diaphragm which bends under the action of the pressure exerted externally on the hydrophone, and a second chamber open onto the outside environment and communicating with the first chamber by a channel. The switch has a push element which is arranged opposite the flexible diaphragm closing the first chamber so as to be actuated thereby for a predetermined outside pressure, the detection unit is arranged in the second chamber, so that at least one of the electrical wires is connected to the switch in the first chamber, so that release of the switch by action of the flexible diaphragm inhibits the response of the sensitive element.
The hydrophone comprises for example a block made of an acoustically transparent material in which the detection clement of the second chamber is embedded.
According to a first variant, the second chamber is separated from the first chamber by an inner partition including at least one channel allowing passage of at least one electrical wire connecting the switch to the detection unit, and the inner wall of the body on the first chamber side is, for example, designed to limit deformation of the diaphragm towards the inside of the first chamber.
According to another variant, the second chamber is separated from the first chamber by an inserted rigid plate made of a dielectric material which is fastened to the inside of the body, this plate carrying the switch and being suited to bring it into electrical contact with at least one electrical wire connecting the switch to the detection unit.
This rigid plate is, for example, an insulating plate provided with conducting tracks allowing connection of the terminals of the switch to the electric wires of the detection unit.
The hydrophone comprises, for example, a ring made of a plastic or metallic material, arranged in the first chamber between the diaphragm and the rigid plate, which is suited to limit deformation of the diaphragm towards the inside of the first chamber.
According to a first embodiment example, the detection element in the second chamber comprises a housing of two cups resting against each other, at least one of the cups being provided with a flexible central part or diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the central part of at least one of the cups, electric conductors connected to the electrodes of each sensitive element, and a protective sheath for protection of the detection unit, the switch being connected to at least one electrical conductor.
According to a second embodiment example, the detection element in the second chamber comprises a housing of two cups resting against each other, at least one of the cups being provided with a flexible central part or diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the central part of at least one of the cups, electrical conductors connected to the electrodes of each sensitive element, and a protective sheath for protection of the detection unit, the switch connected to at least one electric conductor being arranged inside the housing so as to be released by at least one diaphragm of the housing bending under the action of the hydrostatic pressure.
According to another example, the detection element comprises an intermediate support arranged between the two cups, the switch being fastened to this intermediate support with a push element thereof facing the central part of one of the cups, and a sealed insulating bushing for passage, towards the outside of the housing, of an electric conductor connected to the switch.
According to another embodiment example, the switch is of the type provided with flexible blades suited to remain in electric contact with one another as long as the flexion of at least one of the diaphragms remains below a fixed threshold.
According to another embodiment, the tubular body of the hydrop hone comprises a fixed part containing the detection unit and the switch, and a moving part closed at a first end by the diaphragm, tightly screwed onto the fixed part, so that the effective space remaining between the diaphragm and the push element allows the switch to be always released at a well-defined pressure.
The hydrophone is therefore preferably mounted in such a way that the moving part of the body is first fitted onto the fixed part by screwing, a predetermined nominal pressure applied to the diaphragm causing bending thereof towards the inside of the body, then fitting of the two parts onto each other by continued screwing until the switch is actuated by the bent diaphragm.
As the case may be, the switch is normally open or normally closed.
The hydrophone, as defined above, is light and compact. Its outer surface is practically free of rough patches likely to disturb fluid flows around the outer surface. The diaphragm provides perfect sealing and insulates the switch electrically. Furthermore, the embodiment and the mounting mode wherein the body is made of two parts that fit into one another allows obtaining a good reproducibility of the hydrophone triggering conditions.